The invention relates to a method and an arrangement for a tightness check of a tank system.
Methods and arrangements for checking the tightness of tank systems have been known for some time in different embodiments. In most embodiments, an overpressure or underpressure is introduced into the blocked tank system and a conclusion is drawn as to tightness or untightness of the tank system based on the pressure gradient which builds up. Methods of this kind are presented, for example, in U.S. Pat. No. 5,890,474.
A method, known as xe2x80x9cNatural Vacuum Leakage Detection (NVLD)xe2x80x9d was presented at the SAE-Toptech-Conference, Indianapolis 1999. In this method, the tank is blocked by blocking means for a pregiven time span after the vehicle is shut off. In a tight tank, an underpressure then develops. In this method, an underpressure switch is provided which changes its switching state starting at a certain threshold value. This switching state is detected. If, within a pregiven time span, no switching operation is detected, then a conclusion is drawn as to the presence of a leak. The idea of this method is to utilize the underpressure, which settles normally over a period of time when switching off the vehicle and blocking the tank, in order to check the tightness of the tank system. Here, it is assumed that the underpressure arises because of a cooling down of the tank.
What is problematic here is that operating states exist wherein the expected cool-down of the tank does not occur in the shutoff phase; instead, even a warming takes place. An example of this is a trip in winter in cold surroundings and a subsequent switchoff of the vehicle in a warm garage.
Furthermore, a method of this kind does not consider different fuel types which have a vapor pressure, especially in ambient temperatures during summer, which lies above the ambient pressure, which is based on the fact that easily volatile components of the fuel boil in the tank. With this boiling operation, no underpressure can build up even with a cooling down of the tank; instead, an overpressure develops.
Whereas one possibly can eliminate the above-mentioned temperature dependency of the pressure present in the tank via the detection of the ambient temperature and so avoid a fault diagnosis, it is practically impossible without additional sensor means to detect in any way the boiling behavior of the fuel used and to consider the same in the diagnosis.
For this reason, the basis of the invention is to eliminate the above-mentioned disadvantages and to provide a method and an arrangement for checking the tightness of a tank system which makes possible a reliable detection of a leak in a manner simple to realize, on the one hand, independently of the ambient temperature and, on the other hand, independently of the fuel type present in the tank.
The advantage of the method of the invention for checking tightness of a tank system of a vehicle is that one can reliably draw a conclusion as to a leak from detecting the pressure trace in the blocked-off tank by means of a pressure sensor independently of whether an underpressure or overpressure develops in the tank. The advantage of detecting the pressure by means of a pressure sensor is especially that overpressures as well as underpressures can be detected by the pressure sensor so that a conclusion can be drawn as to a leak based on an adjusting or non-adjusting underpressure as well as based on an adjusting or non-adjusting overpressure.
In principle, one could detect the time-dependent gradient of the pressure trace with a pressure sensor and, based on this gradient, draw a conclusion as to a leak present in the tank.
An especially advantageous embodiment which does not require such a gradient measurement provides that one draws a conclusion as to a tight tank system when the pressure, which arises in the tank, passes either a pregivable underpressure threshold value or a pregivable overpressure threshold value after the elapse of a pregiven waiting time. In both cases, a conclusion can be drawn as to a tight system because an overpressure or underpressure will adjust when a leak is present.
It is especially advantageous that the method is not limited to the detection of an underpressure as is the case in the above-mentioned NVLD method. Rather, even with ambient influences for which an overpressure develops in the tank, a conclusion can reliably be drawn as to a leak with the method of the invention by utilizing the pressure sensor.
The processing of the pressure sensor signal in a control apparatus or a circuit unit permits a comparison to variable threshold values which are stored in the control apparatus.
Advantageously, it is provided that one draws a conclusion as to a non-tight system only when neither the underpressure threshold value nor the overpressure threshold value is passed within a pregiven time span with several pass-throughs of the above-described method.
The overpressure threshold value and the underpressure threshold value are advantageously fixed in dependence upon parameters, which characterize the ambient influences, such as the ambient temperature or the tank fill level. In this way, a considerable increase in accuracy of the tightness check is made possible.
In an arrangement according to the invention for checking tightness of a tank system of a vehicle, a drivable blocking means for tightly closing the tank system is provided as is a pressure sensor for detecting the pressure present in the tank system and a control unit for driving the blocking means as well as for processing the pressure sensor signals.
The advantage of this arrangement is its simple configuration. The arrangement can, for example, be subsequently provided very easily in existing tank systems because a control unit is present in all modern vehicles. Accordingly, only a pressure sensor need be provided in the tank system and a blocking means.
In vehicles having tank-venting systems, the function of the blocking means can advantageously be assumed by the tank-venting valve.
To achieve the above-mentioned increase in accuracy by detecting ambient influences, sensors for detecting the ambient influences can further be provided, especially sensors for detecting the ambient temperature and a sensor for detecting the tank fill level whose signals can be processed in the control apparatus.